The present invention relates to a controlling apparatus of a solenoid used for driving a printing section of a printing unit (for example, a printing hammer, a wire, and the like for striking a printing element wheel), more particularly to a controlling apparatus of a solenoid for controlling electric current which flows through a solenoid so as not to be largely varied. This type of controlling apparatus, proposed by the same assignee in, for example Japanese Provisonal Publication SHO 61-263776, has been known.
FIG. 1 shows an outline of a printing mechanism of a typewriter to which the present invention is to be applied. A carriage 102 is reciprocally moved along a platen 100. The carriage 102 holds a daisy shaped printing element wheel 104 with a large number of printing elements on the peripheral position thereof. The printing element wheel 104 is connected to a printing element selection motor 106. Behind the printing element wheel 104, a printing hammer unit 108 is supported by the carriage 102. A hammer 110 of the printing hammer unit 108 is advanced and a selected printing element of the printing element wheel 104 is struck via a printing ribbon 114 from the backside of the printing element wheel 104 to a sheet 112 held by the platen 100 and thereby the desired letter is printed on the sheet 112.
The hammer 110 is driven by a solenoid 116 shown in FIG. 2. The solenoid 116 is fixed in a housing 118 of the printing hammer unit 108. Inside the housing 118, the hammer 110 is movably held by guide members 120 and 122. The hammer 110 is tensioned to the retreat position by an elastic spring 124. However, when the solenoid 116 is powered, the hammer 110 advances against the tension force of the spring 124 and strikes the rear surface of the specified printing element of the printing element wheel 104 as described above.
It is preferred by controlling the solenoid current to keep it constant while the hammer 110 is struck so as to stable the impact force. An outline of a typical control circuit for that is shown in FIG. 3.
A power supply to the solenoid 116 is turned on and off by a switching circuit 126. The solenoid current is converted into voltage value by a resister 128 for detecting the current. The resultant voltage is converted into a digital value, hereinafter named the A/D (Analog/Digital) conversion value, by an A/D converter 130 and a reference voltage generating circuit 132. The A/D conversion value is output to a controlling circuit 134. In this example, the A/D converter 130, the reference voltage generating circuit 132, and the controlling circuit 134 are included in a CPU (Central Processing Unit) 134-1. The control circuit 134 of the CPU 134-1 executes a chopping control operation where it compares a digital value, i.e., the A/D conversion value, with a predetermined comparison value and turns on the switching circuit 126 when the digital value is smaller than the comparison value, while the control circuit 134 turns off the switching circuit 126 when the digital value is greater than the comparison value. Thus, the solenoid current becomes a saw shape in one printing cycle, hereinafter named a solenoid drive time period, so that the current becomes stable in accordance with the comparison value.
However, in the above solenoid control operation, when the solenoid drive voltage is unstable, that is in the unstable state, the rise slope of the solenoid current depends on the solenoid drive voltage. Thus, if the process speed of the A/D converter is slow (for example, the A/D coverter incorporated in a CPU is used), the instability of the solenoid drive voltage causes the solenoid current remarkably to deviate.
FIG. 4A shows a chart of the deviation of the solenoid driving current. As apparently shown in the drawing, when the solenoid drive voltage value "V" is low value "Vl", i.e., V=Vl, the solenoid current is also low as shown by the solid line in the drawing. On the other hand, when the solenoid drive voltage value "V" is high value "Vh", i.e., V=Vh, the solenoid current relatively becomes high as shown by the dot line in the drawing. Now, such a phenomenon will be further described hereinafter in detail by referring to FIG. 4B which is an enlarged view of portion "A" of FIG. 4A.
When the solenoid drive voltage "V" is low in one cycle of A/D conversion time period, i.e., V=Vl, the solenoid driving current "Io" is slowly increased. Thus, when it is determined that the A/D conversion value is greater than the comparison value, the solenoid current becomes "Vlmax". When the power supply is turned off, the current decreases to "Vlmin". On the other hand, in the same A/D conversion time period, when the solenoid drive voltage is high, i.e., V=Vh, the rise slope of the solenoid current becomes sharper than that of the low voltage state, i.e., V=Vl. Thus, when it is determined that the A/D conversion value is greater than the comparison value, the solenoid current increases to "Vhmax". When the power supply to the solenoid is turned off, the solenoid current decreases to "Vhmin."
The difference of the amount of the solenoid current results in a deviation due to unstableness of the solenoid drive voltage. Thus, the impact force of each printing operation deviates and thereby the printing quality tends to be unequal.